Ferroelectricity from coupled cooperative Jahn-Teller distortions and octahedral rotations in ordered Ruddlesden-Popper manganates

Density functional theory and group-theoretical methods are used to explore the origin for ferroelectricity in cation ordered LaSrMnO₄ with the Ruddlesden-Popper structure. The equilibrium phase exhibits the polar Pca2₁ space group where small polar displacements of d⁴Mn³⁺ coexist with antiferrodistortive octahedral rotations and Jahn-Teller distortions. We find that the octahedral rotations and Jahn-Teller distortion stabilize the polar structure and induce polar displacements through high-order anharmonic interactions among the three modes, making LaSrMnO₄ a hybrid-improper ferroelectric material. The rotations result from the ionic size mismatch between A cations and Mn whereas the Jahn-Teller distortions are energetically favored owing to the coupling between the local e_g orbital polarization of the two nearest-neighboring Mn cations in the two-dimensional MnO₂ sheets. Our results indicate that anharmonic interactions among multiple centric modes can be activated by cation ordering to induce polar displacements in layered oxides, making it a reliable approach for realizing acentric properties in artificially constructed materials.